1. Field of the Invention
The invention relates in general to a variable gain amplifier, and more particularly to solving an issue caused by an instantaneous change of a gain provided by a variable gain amplifier.
2. Description of the Related Art
Digital television broadcasting has gradually matured with the progressing communication technologies. Digital Video Broadcasting (DVB) specifications are one of the most common standards in the technology field. Based on differences in transmission media and signal types, DVB is further categorized into sub-standards including Digital Video Broadcasting-Cable (DVB-C), Digital Video Broadcasting-Satellite (DVB-S), and Digital Video Broadcasting-Terrestrial (DVB-T).
FIG. 1 shows a partial circuit block diagram of a DVB-C receiving end. A radio-frequency (RF) signal entering this receiving system is amplified by a low-noise amplifier (LNA) 11. An output signal from the LNA 11 is provided to an analog-to-digital converter (ADC) 12 and a filter 14. An automatic gain control (AGC) circuit 13 selectively adjusts a gain that the LNA 11 applies to the RF signal according to the output signal of the ADC 12. A programmable gain amplifier 15 performs an amplification process to the output signal of the filter 14 after receiving the output signal. Similarly, the AGC circuit 17 selectively adjusts a gain that the programmable gain amplifier 15 applies according to the output signal of the ADC 16. Through adjusting the gain with the above feedback mechanism, the amplitude of the signal entering the ADC 16 is adjusted to satisfy a dynamic range of an input signal of the ADC 16. The digital signal generated by the ADC 16 is provided to a digital baseband circuit 18.
In a DVB-C receiving system, the variable range of the gain provided by the LNA 11 is −18 dB to 24 dB, with a smallest unit change being 6 dB. The variable range of the gain provided by the programmable gain amplifier 15 is −5 dB to 22 dB, with a smallest unit change being 3 dB. It is experimentally proven that, when the gain provided by the LNA 11 and/or the programmable gain amplifier 15 changes, and a gain change greater than 0.25 dB is accordingly instantaneously applied to the amplitude of the input signal of the digital baseband circuit 18, an error may be caused in the signal processing result of digital baseband circuit 18.
In addition to the above DVB-C receiving system, other signal processing systems with similar circuit architecture may also encounter the same issue.